[kube-prometheus-stack] explicitly allow a null Prometheus ruleSelector

[ba-work]

What this PR does / why we need it

This PR enables the user to explicitly set a Prometheus CRD's spec.ruleSelector value to null. This is a supported/documented value that prevents the operator from importing any PrometheusRules. You can check this with kubectl explain prometheus.spec.ruleSelector or see it documented here

I encountered a situation where this is required (CRs created and validated, but not imported), and this chart doesn't allow explicitly setting prometheus.prometheusSpec.ruleSelector=null; currently it always coalesces to {}.

I made it also require prometheus.prometheusSpec.ruleSelectorNilUsesHelmValues=false since that seemed like the most intuitive and backwards compatible with current behavior.

Tested with:

helm install -g charts/kube-prometheus-stack \
  --set prometheus.prometheusSpec.ruleSelectorNilUsesHelmValues=false \
  --set prometheus.prometheusSpec.ruleSelector=null

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[jkroepke]

Could you please extend the unit tests in that case, maybe start by copy this file:

https://github.com/prometheus-community/helm-charts/blob/main/charts/kube-prometheus-stack/unittests/prometheus/scrape_config_selector_test.yaml

[ba-work]

Could you please extend the unit tests in that case, maybe start by copy this file:

main/charts/kube-prometheus-stack/unittests/prometheus/scrape_config_selector_test.yaml

done!

[ba-work]

for anyone that happens to stumble on this before its merged or closed, turns out you can get the same behavior but it relies on the way the operator currently handles on config precedence: if there are keys with the same name, the latter wins.

so that means, as of this comment, the following will result ruleSelector: null when the CR is actually applied to the cluster:

helm install -g charts/kube-prometheus-stack \
  --set prometheus.prometheusSpec.ruleSelectorNilUsesHelmValues=false \
  --set prometheus.prometheusSpec.ruleSelector=null \
  --set prometheus.prometheusSpec.additionalConfig.ruleSelector=null

Really, this will make helm generate this:

---
# Source: kube-prometheus-stack/templates/prometheus/prometheus.yaml
apiVersion: monitoring.coreos.com/v1
kind: Prometheus
...
spec:
  ...
  ruleSelector: {}
  ...
  ruleSelector: null # this one wins

[ba-work]

@jkroepke renovate-bot necessitated another version bump. sorry!